Light versus dark in strong-lens galaxies: Dark matter haloes that are rounder than their stars

TL;DR

This study analyzes the shape and density profiles of dark matter and stellar distributions in 11 strong-lens galaxies, revealing that dark matter haloes are rounder than stars and vary with galaxy properties, providing new constraints for galaxy formation models.

Contribution

It presents the first detailed measurements of the shape and alignment of dark matter and stellar distributions in strong-lens galaxies, highlighting differences in halo roundness and alignment based on external shear.

Findings

Dark matter haloes are rounder than stellar distributions.

Halo shapes vary with galaxy radius and lensing image position.

Alignment between dark matter and stars depends on external shear.

Abstract

We measure the projected density profile, shape and alignment of the stellar and dark matter mass distribution in 11 strong-lens galaxies. We find that the projected dark matter density profile - under the assumption of a Chabrier stellar initial mass function - shows significant variation from galaxy to galaxy. Those with an outermost image beyond $\sim 10$ kpc are very well fit by a projected NFW profile; those with images within 10 kpc appear to be more concentrated than NFW, as expected if their dark haloes contract due to baryonic cooling. We find that over several half-light radii, the dark matter haloes of these lenses are rounder than their stellar mass distributions. While the haloes are never more elliptical than $e_{dm} = 0.2$, their stars can extend to $e_* > 0.2$. Galaxies with high dark matter ellipticity and weak external shear show strong alignment between light and dark; those with strong shear ($\gamma \gtrsim 0.1$) can be highly misaligned. This is reassuring since isolated misaligned galaxies are expected to be unstable. Our results provide a new constraint on galaxy formation models. For a given cosmology, these must explain the origin of both very round dark matter haloes and misaligned strong-lens systems.